This invention relates to the field of material forming, and particularly to superplastic forming of materials.
Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits, a property called superplasticity. The usual process involves placing a sheet of material in a die, heating the material to a temperature at which it exhibits superplasticity, and then using a gas to apply pressure to one side of the sheet. Sufficient pressure is applied to strain the material at a strain rate which is within the superplastic range of the material being formed at the selected temperature. This gas pressure creates a tensile stress in the plane of the sheet which causes it to form into the die cavity.
The elongation and thinning characteristics of the material being formed are related to the rate of straining of the material. To shorten forming time and to prevent rupture of the material, it is necessary to use an optimum forming rate throughout the forming operation. As explained in U.S. Pat. No. 4,181,000, this requires continuous adjustment of the forming pressure to account for changes in the blank as it is blown into and against the forming die.
Current superplastic forming technology relies on precalculated pressurization cycles. This approach is inadequate because: (1) the variation in flow properties of the incoming materials can cause significant inaccuracies and irreproducibility in part fabrication, and (2) development of analytical pressure vs time profiles is both costly and requires relatively sophisticated characterization of the materials' superplastic forming properties. To overcome these problems a monitoring device is needed to continuously measure the actual forming strain rate. The monitored strain rate can then be fed back to automatically control the forming operation. However, conventional devices are not applicable because of the high temperature and inaccessibility of the blank being formed. Also, the superplastic blank is soft at the forming temperature and is easily marred by physical contact with the sensor of a monitoring device.